1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having a wide viewing angle characteristic and being capable of producing a high quality display.
2. Description of the Background Art
In recent years, liquid crystal display devices, which are thin and light in weight, are used as personal computer displays and PDA (personal digital assistance) displays. However, conventional twist nematic (TN) type and super twist nematic (STN) type liquid crystal display devices have a narrow viewing angle. Various technical developments have been undertaken to solve the problem.
A typical technique for improving the viewing angle characteristic of a TN or STN type liquid crystal display device is to add an optical compensation plate thereto. Another approach is to employ a transverse electric field mode in which a horizontal electric field with respect to the substrate plane is applied across the liquid crystal layer. Transverse electric field mode liquid crystal display devices have been attracting public attention and are mass-produced in recent years. Still another technique is to employ a DAP (deformation of vertical aligned phase) mode in which a nematic liquid crystal material having a negative dielectric anisotropy is used as a liquid crystal material and a vertical alignment film is used as an alignment film. This is a type of ECB (electrically controlled birefringence) mode, in which the transmittance is controlled by using the birefringence of liquid crystal molecules.
While the transverse electric field mode is an effective approach to improve the viewing angle, the production process thereof imposes a significantly lower production margin than that of a normal TN type device, whereby it is difficult to realize stable production of the device. This is because the display brightness or the contrast ratio is significantly influenced by variations in the gap between the substrates or a shift in the direction of the transmission axis (polarization axis) of a polarization plate with respect to the orientation axis of the liquid crystal molecules. It requires further technical developments to be able to precisely control these factors and thus to realize stable production of the device.
In order to realize a uniform display without display non-uniformity with a DAP mode liquid crystal display device, an alignment control is necessary. An alignment control can be provided by, for example, subjecting the surface of an alignment film to an alignment treatment by rubbing. However, when a vertical alignment film is subjected to a rubbing treatment, rubbing streaks are likely to appear in the displayed image, and it is not suitable for mass-production.
Another approach proposed in the art for performing an alignment control without a rubbing treatment is to form a slit (opening) in an electrode so as to produce an inclined electric field and to control the orientation direction of the liquid crystal molecules by the inclined electric field (e.g., Japanese Laid-Open Patent Publication Nos. 6-301036 and 2000-47217). However, the present inventors reviewed these publications and found that with the methods disclosed therein, the orientation in regions of the liquid crystal layer corresponding to the openings in the electrode is not defined, whereby the orientation of the liquid crystal molecules is not sufficiently continuous, and it is difficult to achieve a stable orientation across each pixel, resulting in a display with non-uniformity.
In view of this, an inventive entity that includes some of the present inventors proposed another approach (Japanese Patent Application No. 2000-244648), in which a predetermined electrode structure including openings and a solid portion is formed on one of a pair of substrates opposing each other via a liquid crystal layer therebetween, so that a plurality of liquid crystal domains, each of which takes a radially-inclined orientation, are formed in the openings and the solid portion by inclined electric fields that are produced at the respective edge portions of the openings.
However, the present inventors have found that the display quality may not be improved sufficiently only by providing an electrode structure as disclosed in this patent application. This is due to an electric field produced in the vicinity of the edge of a bus line (herein, the term “bus line” is used to refer collectively to a group of interconnection lines) exerting an orientation-regulating force that is not matched with the orientation-regulating force exerted by the inclined electric field produced at the edge portion of the opening.